This application is based on, and claims priority to, Japanese application number 10-171828, filed Jun. 18, 1998, in Japan, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical amplifier and method for amplifying optical signal light using an optical fiber doped with a rare earth element, and especially for amplifying optical signal light in a long wavelength band.
2. Description of the Related Art
With the advancing development of multimedia networks, demand for information is drastically increasing. Therefore, trunk optical transmission systems, which have relatively high information transmission capacity, will be required to have even higher information transmission capacity and will be required to form flexible networks.
To provide higher transmission capacity, wavelength division multiplexing (WDM) optical transmission systems are being used. The commercialization of WDM optical transmission systems has already been advanced mainly in North America.
Moreover, WDM optical amplifiers have been used to amplify WDM optical signals. A WDM optical amplifier can collectively amplify signal lights having two or more different wavelengths in, for example, a wavelength range of 1.53 to 1.57 xcexcm (hereinafter referred to as the xe2x80x9cconventional bandxe2x80x9d). Therefore, the use of WDM optical amplifiers in a WDM optical transmission system can enable high-capacity, long-distance optical transmission with a relatively simple configuration.
Furthermore, by expanding the wavelength band of an optical amplifier, a system has been proposed which employs a long wavelength band, for example, of 1.57 to 1.62 xcexcm (hereinafter referred to as the xe2x80x9clonger wavelength bandxe2x80x9d), as a new transmission band.
The following is a description of the use of an optical amplifier employing an erbium doped fiber (EDF) for the amplification of signal light in the longer wavelength band.
FIG. 1 is a graph showing the gain per unit length versus wavelength characteristics of an EDF corresponding to the degrees of population inversion (ranging from 0.0 to 1.0). As shown in FIG. 1, in the conventional band, the gain characteristic of an EDF is flat in the case that the degree of population inversion is 70% or so. Namely, the gain of an EDF in the conventional band is dominant.
Conversely, in the longer wavelength band, the gain characteristic of an EDF is flat in the case that the degree of population inversion is low, namely, 40% or so. Thus, the gain of an EDF in the longer wavelength band is dominant. Therefore, in the case of optical amplification of signal light in the longer wavelength band, an excitation light of a wavelength in a 0.98 xcexcm band or a 1.48 xcexcm band is supplied to the EDF by setting the degree of population inversion at a low level. In this case, the amplification factor per unit length of the EDF decreases in principle because of the low degree of population inversion.
In the forward excitation case of supplying the excitation light from a signal light input terminal of the EDF to a signal output terminal thereof, the degree of population inversion corresponds to the excitation light power. Therefore, the degree of population inversion is high at the signal light input terminal, but is low at the signal light output terminal. In the backward excitation case of supplying an excitation light from the signal light output terminal of the EDF to the signal input terminal thereof, there is a relation between the degrees of population inversion at the signal input and output terminals thereof opposite to that of the forward excitation case.
Thus, generally, a required total gain of the EDF in the case of the optical amplification of signal light in the longer wavelength band is obtained by elongating a conventional EDF used for the optical amplification of signal light of the conventional band, to thereby lower the degree of population inversion. This can be understood by referring to FIG. 2, which illustrates gain distribution in the longitudinal direction of an EDF.
Further, when the degree of population inversion is set at a low level, the absorption of excitation light is increased. For example, if the wavelength of the excitation light is in the 1.48 xcexcm band in FIG. 1, the gain per unit length of the EDF is about xe2x88x920.2 dB when the degree of population inversion is 0.4. This indicates that, in such a case, the excitation light is more likely to be absorbed in the EDF, as compared with the case where the gain per unit length thereof is about 0 dB when the degree of population inversion is 0.7. When the excitation light is largely absorbed therein, the absorption of excitation light is performed at a biased position in the EDF. Consequently, the excitation power propagation efficiency in the longitudinal direction of the EDF is reduced. Thus, the optical amplification of signal light in the longer wavelength band has a feature that the excitation efficiency of the entire EDF is limited to a low value in comparison with that of the entire EDF in the case of the optical amplification of signal light in the conventional band. A conventional optical amplifier for amplifying signal light of a long wavelength band having such a feature is described in, for example, the article titled xe2x80x9cGain Flattened Er3+ Doped Fiber Amplifier for a WDM Signal in the 1.57-1.60 xcexcm Wavelength Region,xe2x80x9d Ono et al., IEEE Photon. Tech. Lett., Vol. 9, pp. 596-598, May, 1997.
FIG. 3 is a diagram showing such a conventional long wavelength band optical amplifier. In the optical amplifier of FIG. 3, an incident longer wavelength band signal light Ls passes through an optical isolator 21 and is multiplexed with an excitation light Lp1 emitted from an excitation light source 41 by a wavelength division multiplexing (WDM) coupler 31. Then, the multiplexed light enters an EDF 1. An excitation light Lp2 is emitted from an excitation light source 42. At an emitting terminal of EDF 1, excitation light Lp2 is multiplexed by a WDM coupler 32 and is then propagated through EDF 1 in the opposite direction than excitation light Lp1, thereby contributing to optical amplification. The longer wavelength band signal light Ls passes through WDM coupler 32 and optical isolator 22 after passing through EDF 1. Then, signal light Ls is emitted from the amplifier.
FIG. 4 is a diagram showing another conventional long wavelength band optical amplifier. Such a long wavelength band optical amplifier is described, for example, in the article titled xe2x80x9cAmplification Characteristics of 1.58 xcexcm Band Er3+ Doped Fiber Amplifier,xe2x80x9d Ono et al., TECHNICAL REPORT OF IEICE, Vol. OCS97-5, pp. 25-30, 1997.
In the optical amplifier of FIG. 4, an incident longer wavelength band signal light Ls passes through an optical isolator 21. A forward excitation light Lp1 is emitted from an excitation light source 41. Signal light Ls and excitation light Lp1 are multiplexed by a WDM coupler 31. Then, the multiplexed light enters a pre-stage EDF 11. After passing through pre-stage EDF 11, the longer wavelength band signal light Ls passes through an optical isolator 23 and enters a post-stage EDF 12. Further, a backward excitation light Lp2 is emitted from an excitation light source 42. Excitation light Lp2 enter post-stage EDF 12 through a WDM coupler 32. Excitation light Lp2 propagates through post-stage EDF 12 in the opposite direction than excitation light Lp1, and thereby contributes to the optical amplification by the post-stage portion. Then, the longer wavelength band signal light Ls passes through WDM coupler 32 and an optical isolator 22 after passing through post-stage EDF 12. Finally, signal light Ls reaches the emitting terminal of the amplifier. In this case, one of wavelengths (ranging from 960 to 1000 nm) in the 0.98 xcexcm band is used as that of the forward excitation light Lp1. Moreover, one of wavelengths (ranging from 1450 to 1490 nm) in the 1.48 xcexcm band is used as that of the backward excitation light Lp2. With this configuration, a low noise optical amplifier is realized.
FIG. 5 is a diagram showing another conventional long wavelength band optical amplifier. Conventional long wavelength band optical amplifiers, such as that in FIG. 5, are, described, for example, in the article titled xe2x80x9cLow Noise Operation of Er3+ Doped Silica Fiber Amplifier around 1.6 xcexcm,xe2x80x9d Massicott et al., Electron. Lett., Vol. 28, pp. 1924-1925, September 1992, and U.S. Pat. No. 5,500,764 Official Gazette.
In the optical amplifier of FIG. 5, an incident longer wavelength band signal light Ls is multiplexed with conventional band signal light Lp3 through a multiplexer 5, passes through an isolator 21, and is then multiplexed at a WDM coupler 31 with an excitation light Lp1 emitted from an excitation light source 41. Then, the multiplexed signal light Ls enters in EDF 1. At an emitting terminal of EDF 1, an excitation light Lp2 emitted from an excitation light source 42 is multiplexed at a WDM coupler 32. Excitation light Lp2 propagates through EDF 1 in the opposite direction than excitation light Lp1, and thereby contributes to amplification. The longer wavelength band signal light Ls passes through WDM coupler 32 and an optical isolator 22 after passing through EDF 1. Finally, signal light Ls reaches the emitting terminal of the amplifier. This optical amplifier improves the excitation efficiency by adding the conventional band signal light Lp3 to the signal light Ls at a low level. Namely, the more the excitation light having a wavelength close to that of signal light is used, the higher the excitation efficiency. Therefore, the conventional band light is used as excitation light for amplifying the longer wavelength band signal light.
However, the conversion efficiency of the conventional amplifier in FIG. 3 is only 37.7% or so when the gain flattens in the case that the wavelength of the excitation light is set at, for example, one of wavelengths of 1450 to 1490 nm. Therefore, this conventional optical amplifier has a problem that properties, such as excitation efficiency and noise factor, are inferior to those in the case of optical amplification of signal light of the conventional band.
To cope with this problem, the conventional optical amplifier in FIG. 4 reduces the length of pre-stage EDF 11, increases the length of post-stage EDF 12 and uses the 0.98 xcexcm band excitation light, which has good noise characteristics, for the pre-stage portion amplification. Thus, the noise level in the case of this conventional optical amplifier is low, as compared with the optical amplifier of FIG. 3.
However, although the optical amplifier in FIG. 4 is effective in reducing noise, this optical amplifier has problems in that the excitation power transmission efficiency is low and that the excitation efficiency is still low.
In contrast, the optical amplifier in FIG. 5 obtains high excitation efficiency by supplying the conventional band light Lp3 to EDF 1, and reduces the energy consumption thereof. However, this conventional optical amplifier has problems in that an additional light source for generating the conventional band light is required and that active optical parts, such as a light source, are expensive and thus the cost of the amplifier is increased.
Accordingly, it is an object of the present invention to provide a low-cost optical amplifier and optical amplification method which realize high excitation efficiency optical amplification of signal light of new bands, such as the longer wavelength band, by adding only passive optical parts to a conventional doped fiber optical amplifier.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Objects of the present invention are achieved by providing an optical amplifier for amplifying signal light of a predetermined wavelength band. The amplifier includes first and second rare earth element doped fibers cascaded together so that signal light travels through the first rare earth element doped fiber and then through the second rare earth element doped fiber. At least one excitation light source generates excitation light, and at least one excitation light supplying device supplies the excitation light from the excitation light source to the first rare earth element doped fiber. A laser oscillation light generating device laser oscillates spontaneous emission light of a predetermined wavelength band among spontaneous emission lights generated in the first rare earth element doped fiber, so that laser oscillation is generated. The laser oscillation light generating device then supplies light generated by the laser oscillation to the second rare earth element doped fiber as an excitation light.
With such an optical amplifier, when a signal light of a predetermined wavelength band, for example, the longer wavelength band, enters this optical amplifier, the signal light is sent to the first rare earth element doped fiber doped with, for example, erbium. An excitation light output from the excitation light source is supplied to this first rare earth element doped fiber through the excitation light supplying device. Then, a spontaneous emission light is generated, and the signal light is amplified. The laser oscillation light generating device oscillates a light of the predetermined wavelength band, for example, the conventional band, among the generated spontaneous emission lights, so that laser oscillation is generated. Subsequently, the light generated by the laser oscillation is supplied to the second rare earth element doped fiber.
The signal light having passed through the first rare earth element doped fiber enters in the second rare earth element doped fiber. Light generated by the laser oscillation serves as an excitation light in the second rare earth element doped fiber. Thus, the signal light is amplified with high excitation efficiency.
Consequently, the high excitation efficiency amplification of signal light of the longer wavelength band is achieved without adding active optical parts, such as a light source, differently from the aforementioned conventional long wavelength band optical amplifiers. Moreover, the cost of the optical amplifier is reduced.
The laser oscillation light generating device may be, for example, a fiber resonator type device or a traveling-wave type optical oscillator. In the case that the laser oscillation light generating device is the fiber resonator type device, the laser oscillation light generating device includes a pair of wavelength selective reflection type optical devices, one at each end of the first rare earth element doped fiber, which can reflect the spontaneous emission light of the predetermined wavelength band and transmit the signal light and the excitation light. With such a configuration, the spontaneous emission light of the predetermined wavelength band generated in the first rare earth element doped fiber is reflected by the wavelength selective reflection type optical devices to resonate, so that laser oscillation, occurs in the first rare earth element doped fiber.
In the case that the laser oscillation light generating device is a traveling-wave type optical oscillator, the laser oscillation light generating device has a branching portion for branching a part of light emitted from the signal light emitting terminal of the first rare earth element doped fiber, and a wavelength selection portion for receiving the branched light from the branching portion and for selecting and emitting only a spontaneous emission light of the predetermined wavelength band. Moreover, the laser oscillation light generating device has a multiplexing portion for entering a light emitted from the wavelength selection portion, in a signal light incidence terminal of the first rare earth element doped fiber.
With this configuration, a part of light emitted from the first rare earth element doped fiber is branched at the branching portion. A spontaneous emission light of the predetermined wavelength band included in the branched light is selected by the wavelength selection portion and is then sent to the signal light incidence terminal of the first rare earth element doped fiber through the multiplexing portion. Thus, the spontaneous emission light of the predetermined wavelength band becomes traveling waves. Consequently, laser oscillation occurs.
The optical amplifier may be a forward excitation type device in which the excitation light supplying device is placed at the signal light incidence terminal side of the first rare earth element doped fiber. Alternatively, the optical amplifier may be a bi-directional excitation type device in which excitation light supplying devices are placed at the signal light incidence terminal side of the first rare earth element doped fiber and at the signal light emission terminal side of the second rare earth element doped fiber, respectively.
Moreover, it is preferable that the optical amplifier of the present invention has an excitation light reflecting device for reflecting the excitation light and for transmitting the signal light. Practically, in the case that the optical amplifier of the present invention is of the forward excitation type, the excitation light reflecting device is provided at the signal light emission side of the second rare earth element doped fiber. In the case that the optical amplifier of the present invention is of the bi-directional excitation type, the excitation light reflecting device is provided between the first and second rare earth element doped fibers.
As a result of providing the excitation light reflecting device in the optical amplifier of the present invention, the excitation light emitted from the excitation light source is propagated reciprocatingly through the first and second rare earth element doped fibers, so that the conversion efficiency of the excitation is enhanced. Consequently, the present invention provides high excitation efficiency optical amplifier.
Furthermore, in the case of the optical amplifiers of the forward excitation type and the bi-directional excitation type, it is preferable that a third rare earth element doped fiber be cascaded with the signal light incidence terminal side of the first rare earth element doped fiber. In the optical amplifier of such a configuration, an incident signal light is first sent to the third rare earth element doped fiber. Then, the third rare earth element doped fiber is supplied with a forward excitation light of relatively high power and thus amplifies a signal light at a high degree of population inversion. Then, the signal light and excitation light having passed through the third rare earth element doped fiber are sent to the first rare earth element doped fiber. Thus, the signal light is amplified.
Moreover, the laser oscillation of spontaneous emission light of a predetermined wavelength band occurs. Then, the laser oscillation light and the signal light enter the second rare earth element doped fiber, so that the signal light is more greatly amplified. Consequently, the low level signal light is effectively amplified at the pre-stage portion. Thus, noise is reduced in the entire optical amplifier.
Further, preferably, the optical amplifier has a light intercepting device provided between the first and third rare earth element doped fibers for intercepting a light being propagated from the first rare earth element doped fiber to the third rare earth element doped fiber.
When among spontaneous emission light generated in the first rare earth element doped fiber, a component being propagated in a direction opposite to the direction of propagation of the signal light enters in the third rare earth element doped fiber, the signal-light gain of the third rare earth element doped fiber decreases. Thus, the population inversion in the third rare earth element doped fiber, namely, the gain thereof, is maintained at a high level by using the light intercepting device to intercept the spontaneous emission light propagating in a direction opposite to the direction of this signal light. Consequently, the noise characteristics of the entire optical amplifier are further improved.
According to another aspect of the present invention, there is provided an optical amplifier for amplifying a signal light of a first wavelength band and a signal light of a second wavelength band. The amplifier includes a first optical amplifying device for amplifying the signal light of the first wavelength band by using a rare earth element doped fiber, a second optical amplifying device for amplifying the signal light of the second wavelength band by using a rare earth element doped fiber, a light branching device for branching a part of the signal light amplified by the first optical amplifying device, and a branch light supplying device for supplying a branch light from the light branching device to the second optical amplifying device as an excitation light. With such a configuration, when a signal light of a first wavelength band, such as the conventional band, and a signal light of a second wavelength band, such as the longer wavelength band, are amplified by using a rare earth element doped fiber, a part of signal light of the conventional band amplified by the first optical amplifying device is branched by the light branching device and is then supplied to the second optical amplifying device through the branch light supplying device. This branch light sent from the first optical amplifying device serves as an excitation light for the second optical amplifying device. Thus, the signal light of the longer wavelength band is amplified with high excitation efficiency. Consequently, the present invention provides an optical amplifier having excellent amplification characteristics at a low cost.
According to still another aspect of the present invention, there is provided an optical amplification method for amplifying a signal light of a predetermined wavelength band by using a rare earth element doped fiber. The method includes (a) oscillating a spontaneous emission light of a predetermined wavelength band among spontaneous emission lights generated in the rare earth element doped fiber, so that laser oscillation is generated and (b) amplifying the signal light of the predetermined wavelength band by supplying light generated by the laser oscillation to a rare earth element doped fiber as an excitation light.
According to yet another aspect of the present invention, there is provided an optical amplification method for amplifying a signal light of a first wavelength band and a signal light of a second wavelength band by first and second amplifying devices, respectively, each of which uses a rare earth element doped fiber. The method includes (a) branching a part of signal light amplified by the first optical amplifying device, and (b) supplying the branched part to the second optical amplifying device as an excitation light.